1. Field of the Invention
The present invention generally involves support systems used to immobilize and position catalyst particles to provide improved catalytic activity and enhanced contact between gases and catalyst particles. More particularly, the present invention relates generally to catalysts used to reduce the emission of pollutants.
2. Description of Related Art
Oxidation catalysts are used to reduce the emission of pollutants in automotive and industrial process exhaust gases. The noble metals are the most commonly used catalysts for exhaust treatment because of their activity for carbon monoxide and hydrocarbon oxidation and resistance to being oxidized (Jung, H. J. and E. R. Becker, "Emission Control for Gas Turbines," Platinum Metals Rev., 31(4), 162-170, 1987). With rising environmental concern and more stringent emission standards, the demand for these catalysts is rapidly increasing. As a result, there is a need to not only increase the effectiveness and efficiency of the catalysts in reactors, but also to reduce the amount of catalyst required for acceptable operation.
Presently, automotive emissions are treated either by the monolith or the pellet bed. In both types of converter, mass transfer limits the rate of pollutant conversion. The monolith is a metal or ceramic battery of channels of various shapes and sizes (Hegedus, L. L., "Effects of Channel Geometry on the Performance of Catalytic Monoliths," Symposium on Catalytic Approaches to Environmental Control Presented Before the Division of Petroleum Chemistry, Inc., American Chemical Society: Chicago Meeting, August 26-31, 1973; Pereira, C. J., G. Kim, and L. L. Hegedus, "A Novel Catalyst Geometry for Automobile Emission Control," Catal. Rev.--Sci. Eng., 26 (3 & 4), 503-523, 1984). The interior surfaces of the channels are coated with alumina support and noble metal catalyst. The pellet bed converter is a fixed bed of various sizes of spherical or cylindrical ceramic pellets impregnated with noble metal catalyst (Mondt, J. R., "Adapting the Heat and Mass Transfer Analogy to Model Performance of Automotive Catalytic Converters," Journal of Engineering for Gas Turbines and Power. 109, 200-206, 1987).
Existing practices for impregnating monoliths, zeolites or ceramic support particles with noble metals involve either a liquid washcoat technique, ion-exchange, or sputtering (Satterfield, C. N. Heterogeneous Catalysis in Practice. McGraw-Hill Book Company, New York, 1980). The liquid washcoat method usually involves immersion of the support into a liquid solution of the metal precursor, the evaporation of the solvent, and the thermal decomposition of the precursor. Ion-exchange is similar to the washcoat technique except that the metal is electrically plated from the solution. Sputtering is the vaporization of the metal to condense on the cooled support.
An exemplary sputtering procedure for depositing platinum catalyst onto alumina particles is disclosed in U.S. Pat. No. 4,046,712. The platinum is sputtered onto alumina particles which are larger than 1000 .ANG.ngstroms. After the platinum is deposited on the alumina particles, the resulting catalytic particles are dispersed upon larger alumina particles or other support material.
In addition to the monolith and pellet supports, a fiber supported platinum catalyst has also been made (Nicholas, D. M. and Y. T. Shah, "Carbon Monoxide Oxidation over a Platinum-Porous Fiber Glass Supported Catalyst." Industrial and Engineering Chemistry, Product Research and Development, Vol. 15, No. 1, 1976, pp. 35-40; and Nicholas, D. M., Y. T. Shah, and I. A. Zlochower, "Oxidation of an Automobile Exhaust Gas Mixture by Fiber Catalysts," Industrial and Engineering Chemistry, Product Research and Development, Vol. 15, No. 1, 1976, pp. 29-35.). In this fiber support system, a glass fibrous filter is impregnated with platinum by the washcoat technique. Another fiber supported catalyst system is disclosed in U.S. Pat. No. 4,399,185. This particular catalytic system utilizes a loosely packed quartz fiber mat which has been coated with colloidal alumina and platinum particles.
The pressure drop experienced by polluted gases as they pass through the catalytic reactor bed is another area where improvement is needed. Pressure drop is caused by the large amount of catalyst and support material and the need to effect high mass transfer. Higher flow rates decrease the pore and boundary layer mass transfer resistances but increase the pressure drop. The pore and boundary layer mass transfer resistances arise because the pore size is larger than the mean free path of the gases flowing through. It would be desirable to reduce the pressure drop as much as possible without sacrificing catalyst activity or efficiency.
As is apparent from the above, there is a continuing need to develop improved catalyst support systems in which the pore and boundary layer mass transfer resistances which occur in presently available systems are reduced or eliminated and in which the pressure drop is reduced. Such catalyst support systems should also be mechanically and thermally stable while still providing optimum contact between reactant and catalyst particles.